1) Methods and Tools for Compiler Construction, Edited by B. Lorho,
Cambridge University press, 1984.
This has a chapter by Jean Claude Heliard of Alsys on how their
separate compilation works.

2) The Rationale for the Design of Ada by Ichbiah & co. (published
recently in the Ada Companion Series.) This is more general, but
does mention some of the work needed in a compiler to support
separate compilation of packages and so on.